The present invention relates to a single crystal material auxiliary melting apparatus and a single crystal material melting method for heating and melting a single crystal material in an auxiliary crucible and for supplying the melt to a main crucible in a single crystal pulling apparatus for manufacturing dislocation-free single crystal of silicon (Si) by Czochralski pulling method.
In general, in a single crystal manufacturing apparatus based on Czochralski pulling method, the pressure in a highly pressure-resistant airtight chamber is reduced to about 10 torr and fresh argon (Ar) gas is supplied into it, and polycrystal in a quartz crucible installed in the lower portion of the chamber is heated and melted. A seed crystal is immersed from above into the surface of the melt. Then, the seed crystal and quartz crucible are rotated and moved up and down, and the seed crystal is pulled so that a single crystal (the so-called ingot) is grown, which comprises an upper cone portion in conical shape with its upper end protruding under the seed crystal, a body portion in cylindrical shape, and a lower cone portion in conical shape with its lower end protruding.
As a conventional method to melt the raw material in an apparatus as described above, a method for supplying raw material to supplement the decrease of the melt in a crucible (hereinafter referred as xe2x80x9cmain cruciblexe2x80x9d) for pulling up single crystal has been proposed. For example, JP-A-55-130894 discloses a method for melting raw material in an auxiliary crucible communicated with a main crucible and for additionally supplying the melt from the auxiliary crucible to the main crucible via a communicating pipe. Also, JP-A-56-164097 describes a method for supplying and melting solid raw material in an auxiliary crucible in a pulling apparatus from outside the pulling apparatus and for additionally supplying the melt from the auxiliary crucible to the main crucible.
In the past, as the methods for heating and melting raw material of single crystal in an auxiliary crucible, resistance heating method and induction heating method have been known. In the resistant heating method, a resistance heater is disposed around the auxiliary crucible, and heat is generated by applying DC voltage on the heater, and the raw material inside is heated via the auxiliary crucible. In the induction heating method, a coil is installed around the auxiliary crucible, and by applying AC voltage on the heater, secondary induction current is generated on the raw material in the auxiliary crucible. Then, Joule heat is generated on the raw material by the secondary induction current, and the raw material is melted.
In the resistance heating method as described above, however, the heat generated by the heater is indirectly transmitted to the raw material inside via the auxiliary crucible, and thermal efficiency is not very high. As a result, melting time becomes longer, and the auxiliary crucible which is made of quartz and is easily deteriorated at high temperature must be heated to unnecessarily high temperature. Further, the auxiliary crucible must be rotated to stir up the raw material, and this requires a system with more complicated structure.
Also, in a conventional example based on the induction heating method as described above, conductivity of silicon is low at low temperature and high at high temperature. This leads to the problem that thermal efficiency is low at the initial heating. Accordingly, when this method is used, induction heating must be started after a part of raw material is melted at the initial heating and supplied into the auxiliary crucible in order to reduce the melting time.
To overcome the above problems, it is an object of the present invention to provide a single crystal material auxiliary melting apparatus and a single crystal material melting method, by which it is possible to improve thermal efficiency and to reduce melting time when the raw material in the auxiliary crucible is heated and melted by induction heating method.
According to a first invention of the present application, when the raw material is at low temperature and its conductivity is relatively low, secondary induction current is generated on a susceptor and heat of the susceptor is transmitted to the raw material in the auxiliary crucible, and when the raw material is at high temperature and its conductivity is relatively high, the susceptor is moved aside and secondary induction current is generated on the raw material in the auxiliary crucible, and heat is generated in the raw material itself.
Specifically, the first invention of the present application provides a single crystal material auxiliary melting apparatus for heating and melting a single crystal raw material in an auxiliary crucible and for supplying the melt into a main crucible, the apparatus comprising:
a conductive susceptor arranged around the auxiliary crucible so that it can be moved in vertical direction;
a coil wound around the susceptor and with high frequency current applied thereon; and
means for moving the susceptor in such manner that heating of the raw material in the auxiliary crucible is started by arranging the susceptor at a high position on the auxiliary crucible and by heating the susceptor with secondary induction current the susceptor is moved aside from the position on the auxiliary crucible after melting of the raw material has been started, and the raw material in the auxiliary crucible is heated by secondary induction current.
The first invention of the present application also provides a single crystal material melting method in a single crystal material auxiliary melting apparatus, which comprises an auxiliary crucible for heating and melting a single crystal raw material and for supplying the melt into a main crucible, a conductive susceptor arranged around the auxiliary crucible so that the susceptor can be moved in vertical direction, and a coil wound around the susceptor and with high frequency current applied thereon, whereby the method comprises the steps of:
starting to heat the raw material in the auxiliary crucible by arranging the susceptor at a high position on the auxiliary crucible and by generating heat on the susceptor with secondary induction current; and
moving the susceptor aside from the high position on the auxiliary crucible after melting of the raw material has been started, and generating heat on the raw material in the auxiliary crucible by secondary induction current.
In a second invention of the present application, in order to attain the above object, a mass of the raw material is placed on an opening of a pipe to block it and a susceptor is arranged at a position no to heat the mass of the raw material and heating is started, and after the melting of the raw material has been started, the susceptor is arranged at a position to heat the mass of the raw material and the raw material above it, and heat is generated on both the susceptor and the raw material by secondary induction current.
Specifically, the second invention of the present application provides a single crystal material auxiliary melting apparatus for heating and melting a single crystal raw material in an auxiliary crucible and for supplying the melt into a main crucible, the apparatus comprising:
a conductive susceptor arranged around the auxiliary crucible so that it can be moved in vertical direction;
a coil wound around the susceptor and with high frequency current applied thereon;
a pipe having an opening to supply the melt from the auxiliary crucible into the main crucible and formed on the bottom wall of the auxiliary crucible;
means for starting to heat the raw material in the auxiliary crucible by generating heat on the susceptor with secondary induction current and by arranging the susceptor at such a height that a mass of the raw material is not heated and the raw material above the mass is heated, while the mass of the raw material blocks opening of the pipe; and
means for generating heat on both the susceptor and the raw material with secondary induction current and by arranging the susceptor at such a height that the mass of the raw material and the raw material above the mass are heated after melting of the raw material has been started.
Also, the second invention provides a single crystal material melting method in a single crystal material auxiliary melting apparatus, which comprises an auxiliary crucible for heating and melting a single crystal raw material and for supplying the melt into a main crucible, a conductive susceptor arranged around the auxiliary crucible in such manner that the susceptor can be moved in vertical direction, a coil wound around the susceptor and with high frequency current applied thereon, and a pipe with an opening for supplying the melt from the auxiliary crucible into the main crucible, the pipe being formed on bottom wall of the auxiliary crucible, whereby the method comprises the steps of:
arranging the susceptor at such a height as not to heat the raw material mass but to heat the raw material above the mass with the raw material mass placed on the opening of the pipe, and starting to heat the raw material in the auxiliary crucible by generating heat on the susceptor due to the secondary induction current; and
arranging the susceptor at such a height to heat the raw material mass and the raw material above the mass after melting of the raw material has been started, and generating heat on both of the susceptor and the raw material due to secondary induction current.
Further, according to a third invention of the present application, in order to attain the above object, when the raw material is at low temperature and its conductivity is relatively low, secondary induction current is generated on the susceptor and heat of the susceptor is transmitted to the raw material in the auxiliary crucible, and when the raw material is at high temperature and its conductivity is relatively high, secondary induction current is generated in both the susceptor and the raw material inside the auxiliary crucible, and heat of the susceptor is transmitted to the raw material in the auxiliary crucible and heat is generated in the raw material itself. Also, by switching over the frequency of the high frequency current to be applied during the melting process, the melting time can be reduced further.
The third invention of the present application provides a single crystal material auxiliary melting apparatus for heating and melting a single crystal raw material in an auxiliary crucible and for supplying the melt into a main crucible, said apparatus comprising:
a conductive susceptor for supporting the auxiliary crucible; and
a coil wound around the susceptor and with high frequency current applied thereon, whereby:
thickness of the susceptor and frequency applied on the coil are selected in such manner than the thickness of the susceptor will be thinner than penetration depth of the secondary induction current.
Also, the third invention of the present application provides a single crystal material auxiliary melting apparatus for heating and melting a single crystal raw material in an auxiliary crucible and for supplying the melt into a main crucible, the apparatus comprising:
a conductive susceptor for supporting the auxiliary crucible; and
a coil wound around the susceptor and with high frequency current applied thereon, whereby:
heating in the auxiliary crucible is started by applying electric current with relatively high first frequency on the coil and electric current with relative low second frequency is applied after melting of the raw material has been started.
Further, the third invention of the present application provides a single crystal material melting method in a single crystal material auxiliary melting apparatus for heating and melting a single crystal raw material in an auxiliary crucible and for supplying the melt into a main crucible, the apparatus comprising a conductive susceptor for supporting the auxiliary crucible, and a coil wound around the susceptor and with high frequency current applied thereon, whereby the method comprises the steps of:
applying electric current with relatively high first frequency on the coil and starting to heat the material in the auxiliary crucible; and
applying electric current of relatively low second frequency after melting of the raw material has been started.